Chapter 14.Acids and Bases

Early attributes of acids and bases (1600's)

Acids BasesTaste sour Taste bitterTurn litmus red Turn litmus blueReact with metals Feel slipperyReact with carbonates React with fats

Arrhenius Acids and Bases

Definitions of Svante Arrhenius, 1884 First working theory about acids and bases

Acids contain hydrogen and produce H1+ ions in water.

Bases contain hydroxide ions (OH1-) and are soluble in water.

Acids and Bases

Arrhenius Acids and Bases

Acids are molecular compounds; a covalent bond attaches the hydrogen ion to the ad-jacent atom.

Ionization, the separation of the molecule into ions, occurs when the molecule dis-solves in water.

Arrhenius Acids and Bases

Bases are ionic compounds; the hydroxide ion exists in the crystal structure of the solid compound.

Dissociation occurs when the ionic solid dissolves in water, releasing the ions to move about.

Arrhenius Acids and Bases

Common Acids:

HCl(aq), H2SO4, H3PO4, HNO3

HC2H3O2 = CH3COOH = acetic acid

Common Bases:NaOH, KOH

Bronsted-Lowry Acids and Bases

The Arrhenius definition has some problems:

It's restricted to water.

It doesn't explain why solutions of some molecular compounds (NH3) and salts (Na2CO3) are basic.

It doesn't explain why some salt solutions are acidic (aqueous Al3+, Fe3+ solutions).

Bronsted-Lowry Acids and Bases

Definitions of Brønsted and Lowry, 1923 Most widely used theory of acids and bases

Acids are proton donors. Bases are proton acceptors.

Reactions:

HCl(aq) + H2O(l) H3O1+(aq) + Cl1-(aq)

H3O1+(aq) + NH3(aq) NH41+(aq) + H2O(l)

Bronsted-Lowry Acids and Bases

Bronsted-Lowry Acids and Bases

Formation of water by the transfer of protons from H3O1+ ions to OH1 ions.

Bronsted-Lowry Acids and Bases

Works in solvents other than water

Solves the base problem:

NH3(aq) + H3O1+(aq) NH41+(aq) + H2O(l)

CO32-(aq) + H3O1+(aq) HCO3

1-(aq) + H2O(l)

Doesn't solve the acid problem;What is it with Al3+(aq) and Fe3+(aq)?

Lewis Acids and Bases

Definitions of Gilbert Lewis, 1923 Most general theory of acids and bases

Acids are electron pair acceptors.

Bases are electron pair donors.

Bronsted-Lowry Acids and Bases

Conjugate acid-base pairs:A reaction between and acid and a base

produces a conjugate acid and a conjugate baseHCl(aq) + H2O(l) H3O1+(aq) + Cl1(aq)

Acid Base Conjugate Conjugate Acid

Base

H3O1+(aq) + NH3(aq) H2O(l) + NH41+(aq)

Acid Base Conj. Conj. Base Acid

Bronsted-Lowry Acids and Bases

Choose the acid, base, conjugate acid, and conjugate base:

HCOOH(aq) + NH3(aq) HCOO1(aq) + NH41+ (aq)

H2PO41(aq) + H2O(l) HPO4

2(aq) + H3O1+ (aq)

H2O(l) + HPO42(aq) + H3O1+ (aq) + PO4

3(aq)

Bronsted-Lowry Acids and Bases

Amphoteric substances can act as both acids and bases:HCOOH(aq) + H2O(l) HCOO1(aq) + H3O1+ (aq)

NH3 (aq) + H2O(l) NH41+ (aq) + OH1(aq)

HPO42(aq) + OH1(aq) PO4

3(aq) + H2O(l)

HPO42(aq) + H3O1+ (aq) H2PO4

2(aq) + H2O(l)

Mono-, Di- and Triprotic Acids

Monoprotic acids can transfer one protonCH3COOH + H2O CH3COO1 + H3O1+

Diprotic acids can transfer two protonsH2CO3 + H2O HCO3

1 + H3O1+

HCO31 + H2O CO3

2 + H3O1+

The first proton transfer is complete before the second one starts.

Mono-, Di- and Triprotic Acids

Triprotic acids can transfer three protonsH3PO4 + H2O H2PO4

1 + H3O1+

H2PO41 + H2O HPO4

2 + H3O1+

HPO42 + H2O PO4

3 + H3O1+

The first proton transfer is complete before the second one starts. The second proton transfer is complete before the third one starts.

Strengths of Acids and Bases

Acids differ in the extent of ionization when they are put in solution

Strong acids ionize completely. There are only a few strong acids.

Weak acids do not ionize completely. Most acids are weak acids.

The equilibrium constant, Ka, is a measure of the strength of an acid.

The Strong Acids

Formula NameHCl(aq) Hydrochloric

acidHBr(aq) Hydrobromic

acidHI(aq) Hydriodic acidHNO3 Nitric acidHClO4 Perchloric acidHClO3 Chloric acidH2SO4 Sulfuric acid*

*first proton

Some Weak Acids

Formula Name Ka

HSO41 Hydrogen sulfate 1.2 x 102

C9H8O4 Acetylsalicylic acid 3.0 x 104

HCOOH Formic acid 1.8 x 104

HC3H5O3 Lactic acid 1.4 x 104

CH3COOH Acetic acid 1.8 x 105

H2CO3 Carbonic acid 4.3 x 107

H2S(aq) Hydrosulfuric acid 1.0 x 107

HCN(aq) Hydrocyanic acid 4.9 x 1010

C6H5OH Phenol 1.3 x 1010

A comparison of the number of H3O1+ ions present in strong acid and weak acid solu-tions of equal concentration.

The Strong BasesSoluble compounds that contain OH1-

Group 1A Hydroxides Group 2A Hydroxides

LiOHNaOHKOH Ca(OH)2

RbOH Sr(OH)2

CsOH Ba(OH)2

The Weak Bases

Ammonia:

NH3 + H2O NH41+ + OH1

Kb = 1.8 x 105

NH41+ + H2O NH3 + H3O1+

Ka = 5.6 x 1010

The Weak Bases

Anions from weak acids:

CH3COO1 + H2O CH3COOH + OH1

CO32- + H2O HCO3

1- + OH1

SaltsA salt is a compound containing a metal or

polyatomic cation, and a nonmetal or polyatomic anion (except OH1).NaCl, NH4Cl, BaSO4, CaCO3, Al2(SO4)3

Neutralization reactions between an acid and a base produce a salt and water.HCl(aq) + NaOH(aq) H2O(l) + NaCl(aq)

2 Al(OH)3(s) + 3 H2SO4(aq) 6 H2O + Al2(SO4)3 (aq)

Hydrolysis of Salts

Hydrolysis is a reaction of a substance with water. Salts may hydrolyze to form H3O1+ or OH1 along with other products.

Hydrolysis of Salts

The salt of a weak acid and a strong base gives a weakly basic aqueous solution.

NaOH + HC2H3O2 NaC2H3O2 + H2O

NaC2H3O2 + H2O HC2H3O2 + OH1 + Na1+

Reestablishes equilibrium between acetate anion and acetic acid.

Hydrolysis of Salts

The salt of a weak base and a strong acid gives a weakly acidic aqueous solution.

NH3 + HCl NH4Cl

NH4Cl + H2O NH3 + H3O1+ + Cl1

Hydrolysis of Salts

The salt of a weak base and a weak acid can give a weakly acidic, neutral, or weakly ba-sic aqueous solution, depending on acid strengths.

NH4C2H3O2 + H2O HC2H3O2 + NH3

The salt of a strong acid and a strong base give a neutral solution.

NaCl + H2O Na1+ + Cl1 + H2O

Hydrolysis of SaltsSome metal ions, if they're small and have a

high charge, give acidic solutions.

Al3+ + 2 H2O AlOH2+ + H3O1+

Keq = 1.4 x 105

Fe3+ + 2 H2O FeOH2+ + H3O1+

Keq = 6.3 x 103

Cr3+ + 2 H2O CrOH2+ + H3O1+

Keq = 1.6 x 104

Net Ionic Equations2 Al(OH)3(s) + 3 H2SO4(aq)

6 H2O(l) + Al2(SO4)3(aq)

2 HCl(aq) + CaCO3(s)

2 CaCl2(aq) + CO2(g) + H2O(l)

Ionic Equations show dissolved ionic sub-stances as ions rather than as compounds.

Net Ionic Equations show only the participat-ing species. "Spectator" ions are not shown.

Net Ionic EquationsIonic Equations2 Al(OH)3(s) + 6 H1+(aq) + 3 SO4

2(aq) 6 H2O(l) + 2 Al3+(aq) + 3 SO4

2(aq)

2 H1+(aq) + 2 Cl2(aq) + CaCO3(s)

Ca2+(aq) + 2 Cl2(aq) + CO2(g) + H2O(l)

Net Ionic Equations (NIE's)2 Al(OH)3(s) + 6 H1+ 6 H2O(l) + 2 Al3+

2 H1+ + CaCO3(s) Ca2+ + CO2(g) + H2O(l)

Self-Ionization of WaterThe self-ionization of water is an acid-base

reaction in which one water molecule trans-fers a proton to another.

2 H2O H3O1+ + OH1 Kw = 1.0 x 1014

Self-Ionization of Water

2 H2O H3O1+ + OH1 Kw = 1.0 x 1014

Kw = ion product constant for water

Kw = 1.0 x 1014 = [H3O1+] [OH1]

In pure water, [H3O1+] = [OH1] = 1.0 x 107M

The relationship between [H3O1+] and [OH1] in aqueous solution is an inverse proportion; when [H3O1+] is increased, [OH1] decreases, and vice versa.

Self-Ionization of WaterAn acidic solution has [H3O1+] > 1.0 x 107 M

[OH1] < 1.0 x 107 M

A basic solution has [OH1] > 1.0 x 107 M[H3O1+] < 1.0 x 107 M

A neutral solution has [H3O1+] = [OH1] = 1.0 x 107 M

Self-Ionization of Water

Examples:

In a 0.015 M solution of HCl, what is the concentration of OH1?Is the solution acidic or basic?

[OH1] is 4.0 x 105. What is [H3O1+]?

Is the solution acidic or basic?

The pH Scale

[H3O1+] can vary over a wide range, and is often low. Often, you need scientific notation to express it. This isn't always convenient.

A simpler way to write [H3O1+] is pH

pH = log [H3O1+]

[H3O1+] = 10pH

Common (base 10) Logarigthms

A logarithm is the power to which a base, such as 10, must be raised to produce a given number.

Number Logarithm

0.010 = 1.0 x 10-2 -2.00 1.0 = 1.0 x 100 0.00

10 = 1.0 x 101 1.00

Coefficient Exponent Characteristic Mantissa

Powers of 10 and their logarithms

0

1

2

3

4

5

6

7

8

0.E+001.E+052.E+053.E+054.E+055.E+056.E+057.E+058.E+059.E+051.E+06

Power of 10

Base 10 logarithm

Common (base 10) Logarigthms

What happens if the coefficient of the number isn’t 1.0?

Number Logarithm

0.050 = 5.0 x 10-2 -1.30 5.0 = 5.0 x 100 0.70

50 = 5.0 x 101 1.70

Coefficient Exponent Characteristic Mantissa

Integers and their logarithms

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 1 2 3 4 5 6 7 8 9 10

Integer

Base 10 logarithm

Common (base 10) Logarigthms

How logarithms simplify mathematics:

2.594 x 103 x 6.022 x 1023 = 1.562 x 1027

log(2.594e3) + log(6.022 e23) = log(1.562e27)

3.4140 + 23.7797 = 27.1937

antilog(27.1937) = 1027.1937 = 1.562 x 1027

Common (base 10) Logarigthms

How logarithms simplify mathematics:

Slide rules use logarithmic scales for multiplication and division.

The pH ScaleAn acidic solution has [H3O1+] > 1.0 x 107 M

pH < 7.0

A basic solution has [OH1] > 1.0 x 107 MpH > 7.0

A neutral solution has [H3O1+] = [OH1] = 1.0 x 107 M

pH = 7.0

A pH meter is used to measure pH values. The pH of vinegar is 2.32 (left). The pH of milk of magnesia in water is 9.39 (right).

The pH Scale

Give the pH for

[H3O1+] = 0.010 M

= 4.2 x 103 M = 1.0 x 107 M = 6.8 x 1010 M = 1.0 x 1012 M

Are the solutions acidic or basic?

The pH Scale

Give [H3O1+] for

pH = 3.00 = 4.50 = 6.85

= 7.00 = 10.75

Are the solutions acidic or basic?

Buffers

A buffer is a solution that resists major changes in pH when acids or bases are added to it.

A buffer containsA weak acid to react with added baseA weak base to react with added acidMost often, the acid and base are conjugate pairs

Buffers

A buffer made of equimolar amounts of a weak acid and its conjugate base will have a pH equal to log Ka.

log Ka = pKa

Adding a acid will shift the pH of the buffer down, adding base will shift the pH of the buffer up.

BuffersExamples:

What is the pH of a buffer made of 0.10 mole of CH3COOH and 0.10 mole of NaCH3COO?

What is the pH of a buffer made with 1.0 mole of ammonia and 1.0 mole of ammonium chloride?

BuffersA buffer is made with 0.10 mole of CH3COOH

and 0.10 mole of NaCH3COO.

What component of the buffer reacts with added H3O1+?

What component of the buffer reacts with added OH1?

Buffers

A buffer is made with 1.0 mole of NH3 and 1.0 mole of NH4Cl.

What component of the buffer reacts with added H3O1+?

What component of the buffer reacts with added OH1?

Acid-Base TitrationsIn an acid-base titration, a measured volume

of an acid or base of known concentration is reacted with a measured volume of a base or acid of unknown concentration.

The reaction is conducted in a way that exactly equimolar amounts of H3O1+ and OH1 are combined.

Acid-Base TitrationsA student titrates 2.00 mL of vinegar (acetic

acid in water) with 15.85 mL of 0.1048 M NaOH. What is the concentration of acetic acid in the vinegar? Calculate both molarity and mass percent. The density of vinegar is 1.05 g/mL.

CH3COOH(aq) + NaOH(aq)

CH3COONa(aq) + H2O(l)